In my earlier U.S. Pat. No. 4,540,016 issued Sept. 10, 1985, U.S. Pat. No. 4,488,571 issued Dec. 18, 1984 and U.S. Pat. No. 4,799,542 issued Jan. 24, 1989, I have described a flooded evaporator system in which a refrigerant is employed together with oil so that control of the oil concentration in the flooded evaporator is important. In more general terms, however, the flooded evaporator is representative of systems which can be controlled or monitored utilizing a sensor which can respond to a parameter or condition of a liquid and which may be subject to high pressures.
For example, pressure switches of earlier designs can make use of a membrane which can be subjected to a one-sided or differential pressure and transmit movement, e.g. via a rod to a device responsive to such movement. Lever systems may be coupled between the rod and, for example, a switch which a pressure state may actuate. Float switches can respond to liquid level and likewise can transmit motion via a rod.
When, however, such devices are utilized in pressure environments, it is important to seal the device against leakage and in such fashion as to withstand the high pressures which may develop. It is not uncommon, for example, to surround the rod with O-ring-type seals, gland seals or the like, to ensure pressure tightness at high pressures.
However, these constructions create a variety of problems. Firstly, friction-type seals around a rod inherently reduce sensitivity because of the friction force retarding movement. Secondly, with certain friction seals like gland seals and O-rings, the friction force is not constant and varies with the pressure to which the sea is subject.
Accordingly, it is desirable to provide a sensor which is free from these disadvantages of earlier sealed systems.
With respect to refrigeration plants and the like, mention may be made of the fact that every refrigeration apparatus comprises two interrelated circulations. A first circulation is provided for a refrigerator and extends from an evaporator (initially in a vapor form) via a liquid separator, a heat exchanger, the compressor, an oil separator and the condenser to return to the evaporator. A second circulation is provided for the lubricating oil which is required by the compressor and as to which a certain concentration must be present in the refrigerant supplied to the compressor and is, therefore, also present in the refrigerant leaving the compressor. This oil is recovered at the oil separator and must be fed back to the refrigerant. A flooded evaporator may have its oil concentration controlled therein by a oil concentration regulator.
I have found that a certain oil content in the refrigerant can even improve the evaporator heat exchange as long as this oil concentration is not excessive. Generally, the oil concentration should be 5 to 20%.
To prevent an excessive increase in the oil concentration in the refrigerant boiling of the evaporator and, therefore, problems with insufficient oil in the compressor, flooded evaporator systems can operate with a recycle of oil-enriched refrigerant from the evaporator to the compressor.
To supply a mixture free from liquid refrigerant and consisting of refrigerant vapor and oil droplets to the compressor, this mixture is supplied via a heat exchanger to the suction intake of the compressor. The high-pressure liquid from the condenser heats the mixture to evaporate any residual liquid component of the refrigerant so that only the refrigerant vapor and oil will be supplied to the compressor.
While oil deficiency in the compressor is thereby avoided, the oil concentration in the evaporator is found to depend on a variety of operating conditions, for example evaporation pressure and fluctuations in evaporator output, flow velocities and the like s that oil concentration variations can detrimentally affect the overall operation of the system.